1. Field of the Invention
The present invention relates to a medium, in particular to an optical recording medium, on which data can be written with, for example, writing implements, an ink-jet printer, or a sublimation printer.
2. Description of the Related Art
Sophisticated information society in recent years has created strong demands in recording media capable of high storage density, as an alternative to magnetic recording media. As such recording media, compact discs (CDs) and laser discs (LDs) are well known and are widely used. The CDs and LDs are optical read-only media produced by a pressing process. In this process, a plain glass disc known as a glass master is formed into a stamper. The stamper is equivalent to a photographic negative in the sense that it is a reverse of the final compact disc. This stamper is used to make a pressing on a transparent polymer, and the stamped surface is then plated with a reflective film or the like. A plastic coating is finally applied over the film to form an end product. This process permits cost-saving, mass production of media having the same information but is not suitable for a case where a small number of media is to be produced because the stamper is expensive. In this respect, methods to produce in a small scale user-recordable optical recording medium have been developed.
Optical recording medium can generally be classified into two groups: recordable media and erasable media. The former permits recording and reproduction of data while the latter permits erasure of recorded data. Of these, a recordable compact disc with a single substrate is called CD-R. The CD-R is compatible with common read-only CDs and has thus been used widely. The CD-R may be written upon by a user who bought an "empty" disc. Accordingly, it is necessary to provide an indicator for the recorded data so that the user can see what information is stored in the disc.
Users are familiar with a felt-tip with oil based ink to indicate the contents of the disc as a title. Hand-written titles are, however, not preferable for applications where the CD-Rs with data recorded therein are sold to end users. On the other hand, the CD-R may be labeled like a floppy disk with paper or plastic film on which well-designed characters and pictures are printed. However, a large donut-shaped label is required when the entire surface of the disc is to be labeled. It is difficult to position the label positively on the disc for labeling. In addition, the label may be stripped off and be caught by components in a disc player. In this respect other approaches have been desired.
For example, a hydrophilic surface layer may be provided on the side opposite to the light incident surface of the medium. Characters may be written directly on the hydrophilic surface with a pen with an oil-based or water-soluble ink, with a printer, particularly an ink-jet printer using inks containing a solvent. This approach is similar to the one used in commercially available OHP sheets for ink-jet printers. This OHP sheet has a surface coated with a resin containing hydrophilic polymers to fix water-soluble inks. In addition, organic or inorganic fillers are dispersed to form a fine irregular surface, thereby improving print characteristics.
The above mentioned method uses the hydrophilic polymer(s) dissolved in an organic solvent for coating. Accordingly, a drying step is required after application of the solution. This drying step with heat complicates the process and is thus not preferable because the CD-Rs are produced for a short tact time as in typical CDs. In addition, a large-scaled machine is required to reduce the tact time.
Further, the coating film is low in solvent resistance, especially water or alcohol, used for dissolving the hydrophilic polymer. The coating film may be damaged when the surface is wet with water accidentally. As a result, only insufficient properties could be obtained. The organic solvent, if used, may affect badly the working environment or may be a cause of air pollution. Accordingly, it should be avoided to remove the organic solvent through the drying step by environmental considerations. In this respect, there have been strong demands toward a method of forming a film with a solvent-free ultraviolet (UV) curable resin or the like.
However, the inks of the pen or the ink-jet printer may be repelled rather than be absorbed when a coating film of sufficient hardness is formed with a typical UV curable resin. As a result, various measures should be taken.